Corps Orders Safety Evaluation of Fern Ridge Dam

Federal officials say the potential for Fern Ridge Dam to collapse in a large earthquake warrants an in-depth study of its structural integrity and the condition of the sandy base on which it sits.

A recent, routine seismic evaluation of the wide earthen dam on the Long Tom River, 12 miles west of Eugene, indicates that a large-magnitude quake could cause liquefaction in the base of the dam, the U.S. Army Corps of Engineers said Tuesday.

The phenomenon occurs when loose, saturated sands are subjected to seismic shaking, in effect causing earth to move as if liquid.

Liquefaction at Fern Ridge could cause "significant loss of foundation strength and possible damage to the dam itself," said Rich Hannan, chief geological engineer for the corps' Portland district.

The design of the dam and its location both factor into the potential risk, he said.

Built in 1941, Fern Ridge is an earth-filled dam 6,330 feet long and only 44 feet tall. The corps will try to determine the extent of the sand zone beneath the dam as part of its investigation.

The liquefaction threat emerged from relatively new information that suggests the potential for a large-magnitude earthquake off the Oregon Coast along the Cascadia Subduction Zone.

That's not to say people living below the dam should be worried, Hannan said. The preliminary study was based on the worst-case scenario, and the probability of a large subduction zone quake in the next 50 years is relatively low, he said.

But from a design standpoint, the probability is considered high enough to be a concern, he said.

"No one should worry specifically about the effects of damage to Fern Ridge Dam," Hannan said. …

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